1. Field of the Invention
The present invention relates to toner for a developer of electrostatic latent image for use in such applications as electrophotography and electrostatic recording and also to a method for producing the toner as well as to a developer and an image forming method using the toner for a developer of electrostatic latent image.
2. Description of the Related Art
Electrophotography consists of the steps of forming an electrostatic charge image on a photoreceptor, developing the electrostatic charge image using an electrostatic latent image developer which comprises toner for the electrostatic latent image developer (hereinafter abbreviated as toner) , composed of a binding resin, a colorant and toner particles, transferring the toner image obtained to transfer paper, and fixing the transferred image by a heat roll or the like to obtain an image. The electrostatic latent image developers for use in such electrophotography can be roughly divided into two types, that is, a one-component developer utilizing the toner itself which comprises a colorant dispersed in a binding resin and a two-component developer which comprises a mixture of the foregoing toner and a carrier. When copying operations are carried out using these electrostatic latent image developers, from the standpoint of process adaptability such as cleaning of a photoreceptor after image formation thereon for the purpose of subsequent image formation, the electrostatic latent image developer needs to be excellent in such properties as fluidity, transportability, fixability, chargeability, transferability, and cleanability.
Recently, from the space saving standpoint, there has been a demand for the downsizing of an electrophotographic apparatus. In this regard, a system is proposed which omits the cleaning system so that the residual toner is recovered concurrently with the developing (Japanese Patent Application L Laid-Open (JP-A) No. 5-94,113). However, this system, in which the residual toner is recovered concurrently with the developing, suffers from the problem that the recovered toner remains in the developing device without being used for developing because the chargeability of the recovered toner differs from that of the other toner. Therefore, upgrading of the transferability has been required in order to omit the cleaning system.
Recently, because of a growing demand for printing in color, and in particular on-demand printing, in order to produce copies at high speed, there has been reported a method comprising the steps of forming a multicolor image on a transfer belt, transferring the multicolor image at one time to an image fixing material, and fixing the image (JP-A No. 8-115,007). However, this method presents the problem that the overall transfer of the toner is poor because residual toner is found in the primary transfer step, i.e., the transfer of toner from the photoreceptor to the transfer belt, and also in the secondary step, i.e., the transfer of toner from the transfer belt to the image fixing material. Therefore, naturally this method needs a cleaning step. Particularly in the secondary transfer, since the multicolor image needs to be transferred at one time and since the conditions of the image fixing materials vary (for example, thickness, surface property, and others in the case of paper), the improvement of the transferability of the toner itself has become important so as to minimize the influence of these conditions.
In order to improve the transferability of toner, it is necessary to minimize the distribution of chargeability between toner particles or the distribution of non-electrostatic adhesion of the toner particles. From this standpoint, it has been proposed to bring the shape of toner particles closer to a sphere so as to improve the fluidity, chargeability, and transferability of toner (JP-A No. 61-279,864). This is because the adhesion between the toner and the transfer belt or the photoreceptor decreases and therefore the transferability of the toner is improved as the shape of the toner particles approaches a sphere.
However, spherical toner particles present the following problems.
The first problem relates to the method for preparing the spherical toner particles. In a wet process for producing the spherical toner particles, a surfactant or the like is used in order to maintain the dispersibility of the particles. This surfactant is retained on the toner particles as an impurity which causes the chargeability of the toner thus obtained to be inferior to that of the toner prepared by a traditional blending/pulverizing method. In addition, for an unexplainable reason, toner particles produced by a wet process have a larger inter-particle chargeability which results in a broad distribution of the charge of particles. This phenomenon leads to the problems that a sufficient developability cannot be obtained; that non-image areas are developed; that the interior of the developing device is contaminated with toner; and that fogging by the toner increases. Another problem is that, since the nearly spherical toner particles allow the entire surface area of the particles to cause frictional electrification with carrier particles or other toner particles, the trace of impurities derived from the manufacturing process of the spherical toner particles exerts a significant influence and thus further broadens the distribution of the charge. Likewise, because of localized composition on the surface, spherical toner particles produced in a dry process have a broader distribution of charge in comparison with amorphous toner particles.
Furthermore, because of the sphericity of the toner particles, it is impossible to obtain a contacting probability and frictional strength sufficient for electrification by the friction with the carrier.
Accordingly, various attempts have been made in order to improve the chargeability of the spherical toner particles.
For example, it has been proposed to blend the spherical toner particles with an external additive which is a fine powder of an inorganic oxide such as silica in order to further improve the fluidity and the transfer efficiency of the spherical toner particles and to improve the chargeability.
As described above, for the purpose of improving the characteristics, such as fluidity, transferability, and chargeability, of toner, the use of an external additive such as a fine powder of an inorganic oxide has been hitherto known, but it is difficult to make all of the characteristics satisfactory by the external additive.
For example, if a silica fine powder which is generally known as an additive is used, despite the advantage that the fluidity of the toner is improved remarkably by use of the additive, the problem is that there occurs a large difference in chargeability depending on environmental conditions because the additive excessively increases the charge of the negatively chargeable toner in conditions of low temperature and low humidity whereas the additive absorbs moisture and thus decreases the chargeability of the toner in conditions of high temperature and high humidity. For this reason, the use of the additive cannot optimize the chargeability of the toner in both conditions, i.e., conditions of high temperature and high humidity and conditions of low temperature and low humidity, and therefore leads to problems such as poor reproduction of image density, fogging on a photoreceptor, fogging in background, and staining the device interior with the toner. For the purpose of solving these problems, a treatment to hydrophobize the surface of silica particles is proposed by, for example, JP-A Nos. 46-5,782, 48-47,345, 48-47,346, 59-34,539, 59-198,470, and 59-231,550. However, the mere use of the surface-treated inorganic fine power cannot bring about a sufficient effect in chargeability and provides no effect on the spherical toner particles produced by a wet process.
By contrast, titania which is also generally used as an additive is characterized in that the start up of charging is quicker relative to silica and that the distribution of charge is sharp probably because the titania has a low electric resistance. However, the use of the titania brings about disadvantages that a high-level of charge cannot be given to the toner and that the reduction in the amount of charge tends to decrease the reproduction of density and to cause the fogging in background.
In order to solve the problem of the reduction in the amount of charge, a method in which hydrophobic titanium oxide is used as an external additive both to two-component toner and to one-component toner (JP-A Nos. 58-216,252, 60-123,862, and 6-238,847) is proposed. According to this method, hydrophobic titanium oxide is obtained by treating the surface of the titanium oxide with a treating agent such as a silane compound, a silane coupling agent, or a silicone oil. In comparison with traditionally known hydrophilic titanium oxide, the use of the titanium oxide hydrophobized by the treating agent brings about desirable effects in terms of charge and dependence on environment. However, the disadvantage of hydrophobized titanium oxide is that the characteristics inherent to titanium oxide, such as charging speed and the sharpness of the distribution of charge, are remarkably inferior in comparison with traditionally known titanium oxide.
Conventionally,, titanium oxide is obtained mainly from titanium oxide crystals extracted from ilmenite ore by a wet process such as a sulfuric acid process or a hydrochloric acid process. Heating and firing of titanium oxide involved in the wet process naturally cause an inter-particle dehydrating condensation which leads to the formation of a chemical bond to thereby produce many flocculated particles which cannot be easily redispersed by an existing technology. That is, since the titanium oxide taken out as a fine power forms a secondary flocculation and also a tertiary flocculation, the effect of the titanium oxide on improving the fluidity of toner is significantly inferior to that of silica. Particularly, because the fluidity of toner becomes increasingly poor due to the recent use of finer toner particles associated with increased inter-particle adhesion, an additive which is inferior in the fluidity improving effect cannot be used.
In addition, the specific gravity of conventional titanium oxide is larger than that of silica. Therefore, the titanium oxide exhibits another drawback that it does not firmly adhere to the toner surface and tends to be separated from the toner surface. This drawback of the titanium oxide induces contamination of carrier and scratch of photoreceptor surface. As a result, the use of the titanium oxide leads to a poor long-term stability of chargeability and causes degradation of image quality and image defects because the photoreceptor is liable to get contaminated.
Accordingly, various proposals have been made in order to compensate the above-described drawbacks of the titanium oxide while utilizing the characteristics of the titanium oxide that the start up of charging is quick and that the distribution of the charge is sharp.
For example, in order to solve the problems of fluidity and dependence of chargeability on environment at the same time, the use of a mixture of additives, i.e., hydrophobic titanium oxide and hydrophobic silica, has been tried (JP-A No. 60-136,755).
Although this mixture temporarily inhibits the drawbacks of the hydrophobic silica and the hydrophobic titanium oxide, one of the additives tends to exert a dominant influence depending on the state of dispersion. When considering the long-term effect in particular, the stress in a developing device renders the properties of the hydrophobic silica or the hydrophobic titanium oxide more predominant because it is difficult to control the dispersion structure on toner in a stable manner. In short, it is difficult to control the drawbacks of the two additives in a stable manner for a long period of time.
Meanwhile, the addition of a hydrophobic amorphous titanium oxide to toner is proposed (JP-A Nos. 5-204,183 and 5-72,797). The hydrophobic amorphous titanium oxide can be obtained by the hydrolysis of a metal alkoxide or a metal halide by CVD (Collection of Monographs on Chemical Engineering, Vol. 18 (1992), No. 3, pp.303-307).
Although the titanium oxide obtained by hydrolysis solves the problems of the chargeability and the fluidity of toner at the same time, the titanium oxide contains in the particle thereof a large amount of adsorbed water which causes the titanium oxide itself to remain on the photoreceptor at the time of transfer. That is, the strong adhesion between the amorphous titanium oxide and the photoreceptor retains the amorphous titanium oxide alone on the photoreceptor without being transferred. This phenomenon leads to drawbacks such as image voids and formation of scratches on the photoreceptor by the hard titanium oxide at the time of cleaning.
Further, as a method for purifying titanium oxide in a wet process, it is proposed to treat the surface of titanium oxide by hydrolyzing a silane compound in an aqueous medium so as to produce titanium oxide free from flocculation, which is then added to toner (JP-A No. 5-188,633). The titanium oxide which is surface-treated with a silane compound contains a lesser amount of flocculated particles in comparison with conventional hydrophobized silica and therefore the fluidity of toner is improved. However, the titanium oxide does not change the level of charge of negatively charged toner and the dependence on environment at all in comparison with conventional hydrophobized silica. As a result, the purpose of increasing the charge of negatively charged toner and solving the problem of dependence on environment is not sufficiently achieved by the use of the titanium oxide. On the contrary, the speed of charging (admixability of adding toner) and distribution of charge are adversely affected by the titanium oxide.
In order to solve the problems, JP-A No. 6-95,429, 6-102,699, 6-266,156 and others propose the use of a specific binder resin so as to prevent the embedding of an external additive. Further, JP-A No. 6-51,561, 6-208,242, 6-250,442 and others propose the use of a specific charge controlling agent and a specific external additive.
However, none of these proposals brings about a satisfactory effect. Since a full-color developing system in which 4 color images are stacked requires accurate control of toner amounts in developing, the problem of the long-term stabilization of the amount of charge of the toner still remains to be solved.
As described above, an external additive which solves all of the problems, such as fluidity, transferability, and chargeability, has not been obtained. Particularly in the case where spherical toner particles are used, it is necessary to remarkably improve the chargeability of the toner by use of an external additive and, therefore, it is necessary to control more accurately the performance of the external additive than in the case where amorphous toner particles are used. For this reason, the control of the state in which the external additive adheres to the toner is also necessary in addition to the control of the kind and particle diameter of the external additive.
JP-A No. 1-185,654 describes the relationship between the central values indicative of shape of toner and carrier instead of specifying the external additive. According to JP-A No. 1-185,654, if the relationship lies in a specific range, a sharp start-up of charging of toner and a sharp distribution of charge of toner can be obtained. However, one problem involved is that the shape of carrier or toner needs to be close to amorphousness in order to fulfill the condition of JP-A No. 1-185,654, and transfer efficiency becomes poor as the shape of carrier or toner approaches amorphousness. Another problem is that, as the shape of carrier approaches amorphousness, the stress of the amorphous carrier becomes larger than that of spherical carrier in a developing device and the larger stress thus created tends to separate the coating agent from the carrier to an extent that it is difficult for the carrier to exhibit a stable chargeability for a long period of time. In addition, even if the condition of JP-A No. 1-185,654 is met, toner having a shape close to a sphere cannot exhibit a satisfactory chargeability.
The present invention has been made based on the above-described state of prior art.
Accordingly, a first object of the present invention is to provide toner for an electrostatic latent image developer, said toner being excellent in fluidity, transferability, and chargeability and being free from the problems of poor developing, fogging on photoreceptor, and contamination of the interior of developing device, and also to provide a method for producing the toner as well as to provide an electrostatic latent image developer and an image forming method using the toner.
A second object of the present invention is to provide an image forming method which can omit a cleaning step.